A flexible circuit board (flexible printed board) including a resin layer and a wiring made of a metal foil may be used in a bending state, and thus is widely used for various electronic or electrical devices including a movable portion contained in a hard disk drive, a hinge portion or slide portion of a mobile phone, a head portion of a printer, an optical pickup portion, and a movable portion of a notebook PC. In recent times, particularly, with downsizing and thinning of the devices and an improvement of a function thereof, the flexible circuit board requires the bendability which may be folded for compact storage in a limited space and respond to various movements of the electronic devices. Therefore, in order to respond to bending in which a curvature radius of a bend portion becomes smaller or an operation in which folding is frequently repeated, it is necessary to further improve mechanical characteristics including the strength of a flexible circuit board.
In general, the wiring rather than the resin layer is responsible for a defect resulting from poor strength against the repetition of folding or the bending at the small curvature radius. When the wiring becomes difficult to resist the repetition of folding or the bending, the wiring is cracked or broken in part and becomes no longer usable for a circuit board. Therefore, in order to reduce a bending stress applied to the wiring, for example, in a hinge portion, there have been proposed a flexible circuit board in which a wiring is inclined relative to a turning axis (see Patent Literature 1) and a method in which a spiral portion obtained by at least one turn in the turning direction of the hinge portion and the number of turns is increased to suppress a change in diameter of the spiral portion accompanying an open and close operation, to thereby reduce damage (see Patent Literature 2). However, in any of such methods, a design of the flexible circuit board is limited.
In contrast, there has been reported that excellent bendability is exhibited in a case where I/I0>20 is satisfied between an intensity (I) of a (200) plane which is obtained by X-ray diffraction of a rolled surface of a rolled copper foil (X-ray diffraction in thickness direction of copper foil) and an intensity (I0) of a (200) plane which is obtained by X-ray diffraction of a copper fine powder (Patent Literatures 3 and 4). That is, the bendability of the copper foil is improved with the development of a cubic orientation which is a recrystallized texture of copper. Therefore, a known copper foil suitable as a wiring material of the flexible circuit board is one in which the degree of development of a cubic texture is defined based on the parameter (I/I0) described above. There is also a report that an excellent bending strength is obtained when the rolled copper foil has a crystal structure in which an occupied ratio of crystal grains oriented in an orientation allowing a principal slip plane to be active against bending deformation is equal to or larger than 80% in area ratio as observed from a rolled surface (see paragraph 0013 in Patent Literature 5). The description of the specification of this literature may assume that a state in which a cross section of a bent wiring is oriented in {100} is suitable.